In contrast to adult rheumatoid arthritis (RA) little is known about the prevalence, nature and cause of lung function abnormalities in children with juvenile chronic arthritis (JCA). The aim of this study was to determine whether children with polyarticular and systemic onset JCA have lung function abnormalities and if so, whether they are related to pulmonary disease, thoracic and/or muscular involvement. We determined lung function and disability in 31 children with polyarticular and systemic JCA. Respiratory muscle function, thorax expansion and spine mobility were determined in the same patients, as well as in 32 matched healthy children. Peak expiratory flow (PEF) and forced vital capacity (FVC) were significantly reduced in JCA patients, when compared to reference values. Thorax expansion and spine mobility were normal, compared to paired controls. Maximum inspiratory (PI,max) and expiratory (PE,max) pressures were significantly reduced in patients compared to paired control subjects. A positive correlation was found between PE,max and FVC and PEF, an inverse correlation between expiratory pressure and disability. In conclusion, children with polyarticular and systemic juvenile chronic arthritis show a pronounced impairment in respiratory muscle strength, severe enough to cause mild lung function abnormalities and an increase in disability-scores.